hello-algo/codes/java/chapter_backtracking/subset_sum_i_naive.java

/**
 * File: subset_sum_i_naive.java
 * Created Time: 2023-06-21
 * Author: Krahets (krahets@163.com)
 */

package chapter_backtracking;

import java.util.*;

public class subset_sum_i_naive {
    /* 回溯算法：子集和 I */
    static void backtrack(List<Integer> state, int target, int total, int[] choices, List<List<Integer>> res) {
        // 子集和等于 target 时，记录解
        if (total == target) {
            res.add(new ArrayList<>(state));
            return;
        }
        // 遍历所有选择
        for (int i = 0; i < choices.length; i++) {
            // 剪枝：若子集和超过 target ，则跳过该选择
            if (total + choices[i] > target) {
                continue;
            }
            // 尝试：做出选择，更新元素和 total
            state.add(choices[i]);
            // 进行下一轮选择
            backtrack(state, target, total + choices[i], choices, res);
            // 回退：撤销选择，恢复到之前的状态
            state.remove(state.size() - 1);
        }
    }

    /* 求解子集和 I（包含重复子集） */
    static List<List<Integer>> subsetSumINaive(int[] nums, int target) {
        List<Integer> state = new ArrayList<>(); // 状态（子集）
        int total = 0; // 子集和
        List<List<Integer>> res = new ArrayList<>(); // 结果列表（子集列表）
        backtrack(state, target, total, nums, res);
        return res;
    }

    public static void main(String[] args) {
        int[] nums = { 3, 4, 5 };
        int target = 9;

        List<List<Integer>> res = subsetSumINaive(nums, target);

        System.out.println("输入数组 nums = " + Arrays.toString(nums) + ", target = " + target);
        System.out.println("所有和等于 " + target + " 的子集 res = " + res);
        System.out.println("请注意，该方法输出的结果包含重复集合");
    }
}